Specifications

Futurebus+ Bus Support

15.9 Conﬁguring a Futurebus+ Adapter

Figure 15–6 System Control Block

System Control Block

SCB offset

800

810

820

1FE0

1FF0

EXE$GL_SCB

EXE$GL_SCB_RESERVATION

I/O interrupt vectors start at SCB offset 800 (hex)

EXE$GL_SCB_RESERVATION points to a bit−

vector which indicates which SCB entries in the

I/O portion of the SCB are in use. If bit 0 of the

reservation vector is set, then SCB offset 800 is

in use. If bit 1 in the reservation vector is set, then

SCB offset 810 is in use, and so on.

ZK−6731A−GE

Using SDA, you can look at the SCB Reservation bit vector to determine

which interrupt vectors are already in use. Since OpenVMS/AXP starts vector

assignment at SCB offset 800 (hex) and proceeds to higher vector numbers, it

is suggested that for manually conﬁgured Futurebus+ adapters, you start at the

opposite end of the SCB at offset 1FF0 and work your way towards lower vector

numbers, as shown in Figure 15–7. This reduces any chance of a conﬂict with a

previously assigned vector.

15–10